Did you look at flat rate boxes? Medium boxes are about $12 and large are about $14 Also I think postage is usually a little cheaper if you purchase it online.
https://www.prioritymail.com/flatrates.asp
Also maybe try UPS or FedEx- I don't know if they are any cheaper, but it may be worth looking.
EDIT: Here's the link showing what you can save by purchasing the postage online, jsut a few cents but it adds up if you ship a lot:
https://www.prioritymail.com/online_discounts.asp